Electrophotographic paper and method for manufacturing thereof

ABSTRACT

Electrophotographic paper and a method for its production, according to which method a cellulose pulp is slushed, a filler is mixed with the slush, and the slush is formed in the paper machine into a web, which is dried. According to the invention, bivalent or trivalent cations in an amount of at least 0.1% of the dry weight of the pulp are attached to the acid groups of the cellulose pulp before the forming of the web. Thereby a paper web is obtained the surface charge of which is at least approximately 600 V when relative humidity is 40%. The surface charge of the paper according to the invention remains stable within a wide humidity range. For this reason, when the paper according to the invention is used, the need for drying the paper in the printing press is significantly reduced.

[0001] The present invention relates to the production of paper and board. The invention relates in particular to the electrophotographic paper according to the preamble of claim 1, which comprises a fibrous paper web formed from cellulose pulp.

[0002] The invention also relates to a method according to claim 16 for the production of electrophotographic paper from cellulose pulp. According to such a method, the cellulose pulp is slushed, a filler is added to the slush, and the slush is formed in a paper machine into a web, which is dried.

[0003] The invention also relates to the use according to claim 28.

[0004] The importance of the electrophotographic technique in the production of printed matter is increasing. Said technique is particularly well suited for the production of small quantities of printed matter to a customer's order. This is possible because the image to be printed is formed on the photoconductor drum separately for each revolution of the drum. Successive pages may have completely different contents, for which reason it is possible, for example, to print a book to completion in such a manner that the pages arrive on the delivery table of the printing machine in the correct order of pages. Electrophotographic presses and printers are available for both black-and-white printing and four-color printing.

[0005] Electrophotography has long been used as a technique in office copiers and laser printers. The papers used have in this case been uncoated papers, with which there has been obtained a sufficiently high image quality for black-and-white material containing text. For example, in printed advertising material there are a large number of four-color images, and therefore the quality of color images has become an important issue. This, as well as the increased printing speed, increases the demands set on paper.

[0006] In color printing based on electrophotography, the toner particles are transferred from a photoconductor drum to the paper by means of an electric field, negatively charged toner particles transferring onto positively charged paper. The charge is created on the paper surface by means of a first corona. This corona is installed in the machine so that the paper runs between the corona wire and the photoconductor drum.

[0007] The conductivity of paper is a factor important in terms of the success of this printing process. Thus, if the paper is too conductive, the charge discharges from the paper and the toner particles may return to the surface of the photoconductor drum. On the other hand, if the paper is too insulating, a sufficiently strong electric charge does not develop on the paper surface. Furthermore, if the paper is too insulating, static electricity may cause paper sheets to cling to the rolls of the printing machine or cause electric breakdown. To prevent this, the color-printing press may have, after the toner transfer unit, a second corona, which discharges the surface charge of the paper.

[0008] The conductivity of paper varies strongly according to the moisture content of the paper. As is shown in accompanying FIG. 1, the surface charge of commercial electrophotographic papers is in general at its highest (approximately 600-800 V) when relative humidity is approximately 10-20%. When humidity increases to 40%, the surface charge is typically halved or drops to as low as one-tenth of its highest value. Advanced printing machines therefore have pretreatment units wherein the paper is heated in order to adjust the charging capacity of the paper to the correct level. Heating dries the paper, from which there follow warping and waviness. Hot paper is also brittle, which increases breaks in web-fed printing. Preheating of paper is thus not problem-free.

[0009] In advanced presses it is often also possible to adjust the corona wire control currents. However, the finding of the correct settings takes a great deal of time, which reduces the printing press time usable for printing. The finding of the correct settings also causes extra materials costs as toners and papers are wasted. On the other hand, there are on the market also electrophotographic presses in which it is not possible significantly to adjust the parameters of copying.

[0010] It is an object of the present invention to eliminate the disadvantages associated with the current technology and to provide an entirely novel paper or similar fiber product suitable for use as a print medium for the electrophotographic technique.

[0011] The invention is based on the unexpected observation that by attaching as a counter-ion for the carboxylic acid groups in the paper pulp a bivalent or trivalent metal ion in the place of univalent ions it is possible to adjust the resistivity and dielectric properties of the paper into a range especially advantageous for electrophotographic printing. The paper of a novel type according to the invention contains bivalent or trivalent cations in an amount of at least 0.1% of the dry weight of the pulp. Said cation is attached to the acid groups of the cellulose pulp before the forming of the web. The surface charge of the paper web produced is at least approximately 600 V when relative humidity is 40%.

[0012] With the help of the invention there is provided a method for producing printed matter, according to which method the desired image is printed on the surface of a paper or board sheet or web by the electrophotographic technique.

[0013] More specifically, the electrophotographic paper according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

[0014] The method according to the invention for the manufacture of electrophotography paper is, for its part, characterized by what is stated in the characterizing part of claim 16.

[0015] The use according to the invention is characterized by what is stated in claim 28.

[0016] The invention provides considerable advantages. Thus, by binding calcium ions or corresponding multivalent metal ions to carboxylic acid groups as counter-ions for said carboxylic acid groups, a change of two orders can be obtained in the resistivity of the paper without a need to change the structure or the paper (e.g. its coating). It is particularly significant that the surface charge of the paper according to the invention remains quite stable within a wide humidity range. When calcium is used as the counter-ion, the change in the surface charge is less than 25% when relative humidity varies between 20 and 50%. For this reason, when the paper according to the invention is used the need for drying the paper in a press is significantly reduced.

[0017] The invention will be discussed below in greater detail with the help of a detailed description and a number of working examples.

[0018]FIG. 1 shows graphically the surface charges of 5 commercial copier papers and a PET film, as a function of relative humidity; and

[0019]FIG. 2 is a corresponding graphic representation of the change in the surface charge of papers made of two different birch pulps, as a function of relative humidity, the black squares indicating the surface charge of the reference paper containing pulp in the hydrogen form and the circles indicating the surface charge of the paper according to the invention.

[0020] According to the invention, the electrophotographic paper used is a web of material formed from a cellulose pulp wherein at least some of the acid groups contain as the counter-ion a bivalent or trivalent metal ion in order that the surface charge of the paper can be adjusted to a value higher than 600 V when relative humidity is 40%.

[0021] By “web of material” is meant in the present invention a paper or board or a corresponding cellulose-containing material derived from a lignocellulose-containing raw material. “Electrophotographic paper” means the above-mentioned web of material, which contains a filler and which is possibly coated with a pigment layer.

[0022] The wood material in the web of material may be derived from softwood or hardwood. Examples of suitable Finnish species of wood include pine, spruce, birch, aspen and alder. Other species of wood usable as the raw material include eucalyptus, acacia, maple, and mixed tropical hardwood. The invention can also be applied to annual plants, such as grain straw, reed canary grass, reed, and bagasse.

[0023] The web of material may be wood-containing or wood-free, and it is produced in particular from chemical pulp. The pulp may be bleached or unbleached. The material may also contain recycled fibers, in particular recycled office paper or printing paper. The proportion of recycled fibers may be 0-100%.

[0024] It is particularly preferred to use a chemical hardwood pulp having a high hemicellulose content, such as birch pulp. Such a cellulose pulp contains hemicellulose compounds over 10% of the dry weight of the pulp. The chemical pulp may be produced by cooking processes known per se, in alkaline, acid or neutral conditions. Examples to be mentioned include sulfate, sulfite, neutral sulfite and various organosolv processes. The sulfate process (as well as its modified forms, such as extended cooking processes) is regarded as particularly advantageous.

[0025] It is known that chemical cellulose pulp contains carboxylic acid groups, primarily methyl glucuronic groups and, according to the conditions of the cooking and bleaching, also hexenuronic acid groups. According to the invention, the counter-ions of at least these acid groups are replaced by bivalent or trivalent metal ions. Preferably, in papers according to the invention, bivalent or trivalent metal ions constitute most, i.e. at least 50%, preferably at least 70%, especially preferably at least 80%, of the counter-ions of the acid groups.

[0026] The bivalent or trivalent cations are attached to the acid groups of the cellulose pulp by ion exchange. Preferably this is carried out by contacting the cellulose pulp with an aqueous solution of a salt comprising bivalent or trivalent cations, in conditions wherein the salt is at least in part in a dissociated form. The aqueous phase preferably contains salt approximately 0.001-10% by weight, preferably approximately 0.01-5% by weight. The pH value of the ion exchange is approximately 6-10, preferably approximately 6.5-9, and the consistency of the pulp is approximately 0.1-10%. The ion exchange time is approximately 1 min-24 hours and temperature approximately 10-60° C., preferably approximately 15-50° C.

[0027] Most commonly, sodium-containing chemicals, such as sodium hydroxide, are used for the preparation of sulfate pulp, both in the cooking and in the bleaching. For this reason the acid groups in the pulp obtained are mainly in the sodium form. According to a preferred embodiment of the invention, the univalent metals of the acid groups are first removed from the pulp used for the making of the material web, whereafter the pulp is optionally washed and mixed with a salt of a bivalent or trivalent metal in order to replace the hydrogen atoms of the carboxyl groups with bivalent or trivalent metals. The ion exchange may also be carried out by washing the pulp obtained from the pulp making with water that contains bivalent or trivalent metal ions.

[0028] The ion exchange can be implemented as a batch process or a continuous-working process. In practice the ion exchange can be carried out in a separate mixing container or a mixing container/washer combination fitted at a point before the paper machine headbox.

[0029] Cellulose pulp ion exchange is also described in an article by Scallan and Grignon [Scallan, A. M. and Grignon, J., Svensk Papperstidn. 82 (1979) 40-47]. According to the method of Scallan and Grignon, the pulp is first converted to the hydrogen form by soaking the pulp alternately in a dilute acid solution and in a sodium chloride solution, the soaking being concluded in acid conditions. The pulp is thereafter washed with ion-exchanged water, and the pH of the pulp is set at an acid level. The pulp in the hydrogen form is thereafter contacted with a metal salt solution.

[0030] According to a preferred embodiment of the invention, calcium ions are attached to the acid groups by adding to an aqueous suspension of the cellulose pulp a calcium salt so that the hardness of the water rises at least to a level of 10 dH° (which corresponds to a calcium ion concentration of 70 mg/l), whereafter the pH of the suspension is raised to a neutral or mildly alkaline level.

[0031] Preferably the ion exchange is carried out before other paper machine chemicals, such as cationic polymers, retention chemicals and/or fillers, are added to the pulp.

[0032] The water-soluble salts of bivalent or trivalent metals used are their halides, carbonates, sulfates or organic salts. It is also possible to use hydroxides of the metals. Examples of suitable calcium compounds include calcium chloride, calcium hydroxide, calcium ascorbate, and calcium gluconate. Suitable salts of magnesium include magnesium chloride and magnesium sulfate, and suitable salts of aluminum include aluminum sulfate. It is also possible to use mixed salts, such as alum. By ion exchange, there are bound to the acid groups bivalent or trivalent ions in an amount of at least 0.1%, especially preferably 0.15-0.5%, in which case the surface charge of the paper web will be at least approximately 600 V, preferably at least approximately 610 V, and especially preferably at least 620 V, when relative humidity is 40%. The ions bound to the acid groups are preferably calcium, magnesium or aluminum ions or a mixture thereof.

[0033] The ion-exchanged paper pulp is slushed in a manner known per se to a suitable consistency (typically to a solids content of approximately 0.1-1%) and is spread onto the wire, where it is formed into a paper or board web. For the production of electrophotographic paper, before the paper machine there is added to the fiber slush a filler, such as kaolin or precipitated or ground calcium carbonate, in general approximately 1-50%, preferably approximately 5-30% of the weight of the fibers. It is also possible to add to the slush various retention chemicals and cationic polymers, including starch and modified starches.

[0034] The paper web may be uncoated or coated. The grammage of an uncoated web of material typically varies within the range of 30-180 g/m². A coated paper web has in this case preferably a coating pigment layer, wherein the coating pigment is preferably calcium carbonate, kaolin, gypsum, talc, chalk and/or a mixture thereof. The grammage of the web to be coated is 50-250 g/m², and it contains coating approximately 8-35 g/m² of surface.

[0035] According to a preferred embodiment of the invention, the surface charge of the surface can be modified, in particular it can be raised, by applying to the paper surface, for example, by surface sizing or in connection with the coating, a cellulose derivative in an amount of approximately 0.1-20% of the pulp weight, the derivative having a bivalent or trivalent counter-ion. For example, a calcium-form carboxymethyl cellulose can be used as such a cellulose derivative.

[0036] After the drying of the paper web there is obtained a paper which contains a bivalent or trivalent metal ion approximately 0.1-1% of the weight of the fibers (indicated as metal), and the surface charge of which is 600-800 V when relative humidity is approximately 20-40%. The paper typically contains a filler approximately 10-30% by weight of the total weight of the paper.

[0037] The paper according to the invention is suitable for use as a print medium in the production of printed matter by the electrophotographic method. It can be used, for example, in Xeikon four-color electrophotography printing presses [Van Daele, J., Verluyten, L., and Soulliaert, E., Print Media for Xeikon's DCP/32D Digital Color Press, IS&T's NIP12: International Conference on Digital Printing Technologies pp. 382-386].

[0038] The following examples illustrate the invention.

EXAMPLE 1

[0039] A sulfate pulp was prepared in laboratory conditions from pure birch chips. The pulp was oxygen delignified, was chelated with EDTA, and was bleached with peroxide in two steps. The kappa number of the pulp was 7.2, its viscosity 1020 ml/g and its ISO brightness 85.3%.

[0040] The carboxylic acid concentration was assayed by conductometric titration and the uronic acid concentration was assayed with HPLC after a complete hydrolysis performed with enzymes. The methylglucuronic acid concentration was 34 mmol/kg and the hexenuronic acid concentration 72 mmol/kg. Uronic acids constituted 89% of the acid groups in the pulp.

[0041] The pulp was converted to the hydrogen form and thereafter to a sodium, calcium or aluminum form. The ion exchange was carried out by treating the acid-form pulp in aqueous solutions of salts of metal ions. The sodium and calcium ions were introduced in alkaline conditions; in the case of aluminum ions the pH value of the solution was not controlled.

[0042] The metal concentrations in the pulps were: Na pulp: 2672 mg Na/kg; Ca pulp: 2219 mg Ca/kg; and Al pulp: 944 mg Al/kg. Of the available acid groups, at least 75% had been substituted for by metal ions. In general the conversion of the pulp was at least approximately 95% (i.e. 95% of the bonded counter-ions were of the desired ion type).

[0043] The surface charge was determined from the paper samples made from the pulps by using a JCI 155 Charge Decay Test Unit. The charge voltage was 6 kV and the charging time was 7.0 s.

[0044]FIG. 2 shows a comparison between the surface charges of paper sheets in the sodium and respectively the calcium form as a function of relative humidity. As is evident from the said figure, the surface potential of the Ca-form paper is quite uniform within a wide humidity range, whereas the surface potential of the sodium-containing paper drops sharply when relative humidity rises above 40%.

[0045]FIG. 1 presents the surface charge of five commercial electrophotography papers, as a function of relative humidity (rh). The commercial paper grades contain calcium carbonate as a filler, the concentrations being approximately 15-23% by weight. A fact noteworthy in a comparison between the paper according to the invention and these commercial papers is in particular that the surface charge of not a single commercial paper exceeds 400 V when relative humidity is 40%, whereas the surface charge of the paper according to the invention is over 500 V even at an rh value of 50%. This shows that in terms of the surface charge formation it is important that the calcium ions are bonded to the acid groups. A salt of a bivalent or trivalent metal as a filler does not affect the charging of the paper. We have observed this also in laboratory sheet experiments in which calcium carbonate was added to pulp sheets the pulp of which had not been ion exchanged.

EXAMPLE 2

[0046] The acid groups in a chemical cellulose pulp were converted to the calcium form in an industrial process, through the following steps:

[0047] 1) the pulp was washed at a consistency of approximately 1% at a temperature of 45° C. (added NaOH 1% of the dry weight of the pulp)

[0048] 2) CaCl2 was added so that the hardness of the water rose to 10 dH° (70 mg/l Ca²⁺)

[0049] 3) the pulp was washed with tap water

[0050] 4) the pH was adjusted to 5, the pulp was mixed for 10 min

[0051] 5) the pulp was washed carefully to remove the acid and to raise the pH value to 7.

[0052] The bleached pine sulfate initially contained sodium 45 meq/kg and calcium 2 meq/kg. After the treatment the figures were sodium 1 meq/kg and calcium 48 meq/kg. In the bleached birch sulfate the initial situation was sodium 29, calcium 7 and magnesium 35 meq/kg, the final situation was sodium 2, calcium 21 and magnesium 11 meq/kg. 

1. An electrophotographic paper which comprises a fibrous paper web formed from a cellulose pulp, characterized in that it contains, in an amount of at least 0.1% of the dry weight of the pulp, bivalent or trivalent ions bonded to the acid groups of the cellulose pulp, in which case the surface charge of the paper web is at least approximately 600 V when relative humidity is 40%.
 2. The paper according to claim 1, characterized in that the ions bonded to the acid groups are calcium, magnesium or aluminum ions or a mixture thereof.
 3. The paper according to claim 1 or 2, characterized in that the amount of the calcium, magnesium or aluminum bonded to the acid groups is approximately 0.15-0.5% of the dry weight of the fibers.
 4. The paper according to any of claims 1-3, characterized in that the surface charge of the paper is at least 610 V when relative humidity is 40%.
 5. The paper according to any of the preceding claims, which paper contains calcium as the counter-ion for acid groups, characterized in that the change in the surface charge of the paper is at maximum ±25% when relative humidity is 20-50%.
 6. The paper according to any of the preceding claims, characterized in that the cellulose pulp is a chemical hardwood pulp that contains hemicellulose compounds over 10% of the dry weight of the pulp.
 7. The paper according to claim 6, characterized in that the cellulose pulp is birch sulfate pulp.
 8. The paper according to any of the preceding claims, characterized in that the bivalent or trivalent ions are mainly bonded to methylglucuronic acid or hexenuronic acid groups.
 9. The paper according to any of the preceding claims, characterized in that the cellulose pulp contains, in an amount of approximately 0.1-20% of the weight of the pulp, a cellulose derivative having a bivalent or trivalent counter-ion.
 10. The paper according to claim 9, characterized in that the cellulose pulp contains a calcium-form carboxymethyl cellulose.
 11. The paper according to any of the preceding claims, characterized in that it contains a filler 1-40%, preferably approximately 5-30%, of the weight of the cellulose pulp.
 12. The paper according to claim 11, characterized in that the filler is kaolin or precipitated or ground calcium carbonate.
 13. The paper according to any of the preceding claims, characterized in that on the surface of the paper web there is a coating pigment layer.
 14. The paper according to claim 13, characterized in that the pigment is calcium carbonate, kaolin, gypsum, talc, chalk and/or a mixture thereof.
 15. The paper according to any of the preceding claims, characterized in that the grammage of the paper is 50-180 g/m² and it contains coating approximately 0-30 g/m² of surface.
 16. A method for the production of an electrophotographic paper from a cellulose pulp, according to which process the cellulose pulp is slushed, a filler is mixed with the slush, and the slush is formed in the paper machine into a web, which is dried, characterized in that bivalent or trivalent cations in an amount of at least 0.1% of the dry weight of the pulp are attached to the acid groups of the cellulose pulp before the forming of the web.
 17. The method according to claim 16, characterized in that the cellulose pulp used is a chemical pulp made from hardwood.
 18. The method according to claim 16 or 17, characterized in that the cellulose pulp used is birch sulfate pulp.
 19. The method according to any of claims 16-18, characterized in that the bivalent or trivalent cations are attached to the acid groups of the cellulose pulp by ion exchange.
 20. The method according to claim 19, characterized in that the bivalent or trivalent cations are attached to the acid groups by contacting the cellulose pulp with an aqueous solution of a salt comprising bivalent or trivalent cations, in conditions wherein the salt is at least in part in a dissociated form.
 21. The method according to claim 20, characterized in that calcium ions are attached to the acid groups by adding to an aqueous suspension of the cellulose pulp a calcium salt so that the hardness of the water rises to 10 dH°.
 22. The method according to any of claims 16-21, characterized in that the bivalent or trivalent ions are attached to the acid groups of the cellulose pulp before retention chemicals, cationic polymers and/or fillers are added to the cellulose pulp.
 23. The method according to any of claims 16-22, characterized in that the cellulose pulp used is a recycled chemical pulp.
 24. The method according to any of claims 16-23, characterized in that a cellulose derivative having a bivalent or trivalent counter-ion is applied to the paper web in an amount of 0.1-20% of the weight of the cellulose pulp.
 25. The method according to claim 24, characterized in that the cellulose derivative comprises a calcium-form carboxymethyl cellulose.
 26. The method according to any of claims 16-25, characterized in that a filler is added to the slush in an amount of approximately 1-40%, preferably approximately 5-30%, of the weight of the cellulose pulp.
 27. The method according to claim 26, characterized in that the filler used is kaolin or precipitated or ground calcium carbonate.
 28. The use, as the print medium for printed matter to be produced by electrophotographic, a paper web made up of a cellulose pulp that contains, in an amount of at least 0.1% of the dry weight of the fibers of the cellulose pulp, bivalent or trivalent ions bonded to the acid groups of the cellulose pulp, in which case the surface charge of the paper web is at least approximately 600 V when relative humidity is 40%.
 29. The use according to claim 28, characterized in that the bivalent or trivalent ions are calcium, magnesium or aluminum, or a mixture thereof. 